The present invention relates generally to laser printers, and more particularly to a light source for an interlaced printer and optical image bar.
As a matter of definition, an "optical image bar" comprises an array of optical pixel generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. Although there are a variety of applications for such devices and a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing.
One type of image bar is based on the use of total internal reflection electro-optic spatial light modulators, as described in U.S. Pat. No. 4,396,252 to W. D. Turner, hereby incorporated by reference. The modulator comprises a set of laterally separated, individually addressable electrodes, which are maintained closely adjacent a reflective surface of an optically transparent electro-optic element, such as a lithium niobate crystal. In operation, substantially the full width of the electro-optic element is illuminated by a transversely collimated light beam. This light beam is applied to the electro-optic element at a near grazing angle of incidence with respect to its reflective surface, and is brought to a wedge-shaped focus on that surface so that it is totally internally reflective therefrom.
Voltages representing a linear pixel pattern are applied to the individually addressable electrodes, whereby localized fringe electric fields are coupled into the electrooptic element. These fields produce localized variations in the refractive index of the electro-optic element, so the wavefront of the light beam is spatially phase modulated in accordance with the pixel pattern as it passes through the electro-optic element. The process is repeated for a sequence of pixel patterns, with the result that the wavefront of the light beam is spatially modulated as a function of time in accordance with successive ones of those patterns.
For image bar applications of such a modulator, schlieren optics are employed to convert the phase modulated wavefront of the light beam into a corresponding series of optical intensity profiles. If a printing function is being performed, these intensity profiles are in turn used to expose a photosensitive recording medium, such as a xerographic photo receptor, in accordance with the image defined by the successive pixel patterns.
U.S. Pat. No. 4,940,314, issued Jul. 10, 1990, to D. L. Hecht, hereby incorporated by reference, addresses the problem that the effective diameter of the pixels produced by an electro-optic image bar, as measured between their half power points at unity magnification, is approximately one-half the center-to-center spacing of its electrodes. Accordingly, such image bars not only tend to cause image distortion because of spatial quantitization errors, but also characteristically produce inter-pixel intensity nulls.
The patent describes a technique wherein a discrete optical image bar sequentially generates a plurality of independent pixel patterns at different center wavelengths, and a prism system disperses those pixel patterns in accordance with their respective wavelengths to passively increase the spatial addressing capacity of the image bar. The wavelengths of the pixel patterns, the angular dispersion of the prism system, and the length of the optical arm along which the dispersion has effect are selected so that the pixel patterns are laterally offset from one another on the output image plane by a distance that is less than the center-to-center spacing of the pixels of any one of those patterns. As a general rule, the wavelength of the pixel patterns is cyclically or otherwise recurrently varied, so that the pixel patterns are interlaced in accordance with a predetermined lattice-like interlacing pattern.